Sometimes, for diagnostic purpose and/or for radiation treatment planning, the target region of the patient may be imaged using a CT system. For the case in which the target region moves in a periodic motion (e.g., due to breathing), the CT system may be used to determine volumetric images of the target when the target is at different breathing states, so that the volumetric images may be played back as a video stream. One such imaging technique is known as 4D cone beam CT (CBCT). For such purpose, projection images of the target, when the target is at different breathing states, are acquired. A breathing monitoring device is used to determine breathing states of the patient as the CT system acquires the projection images.
After the imaging session, the projection images are then sorted into different sets according to the recorded breathing states of the patient when the corresponding projection images are acquired. For example, the projection images may be sorted according to the phase of the physiological cycle at which they are generated, so that projection images are sorted into different phase bins. After the projection images are sorted, the projection images in each of the phase bin are then used to reconstruct a volumetric image for that phase bins.